By way of example, highly diagrammatic accompanying FIG. 1 recalls the known configuration of a two-reflector active antenna of the offset type, i.e. an antenna of the kind to which the present invention applies.
That antenna uses the optical periscope principle and comprises an active array 1 of small size relative to the direct radiation active array that would be required for radiating a beam of diameter D identical to that which is finally radiated by the offset configuration two-reflector antenna.
The active array 1 is associated in a manner that is conventional for this kind of array with phase adjusting devices 2, and also with amplifiers and filters (not shown), which devices are referred below as "controls" in order to comply with the terminology used in the art.
The beam of diameter d radiated by the active array 1 is initially reflected by a first parabolic reflector 3 which concentrates the beam at its focus F, after which the beam continues to propagate from said focus F to illuminate a second parabolic reflector 4 facing the reflector 3 via the focus F with which it is confocal, thereby finally radiating a beam of parallel rays having a width D.
It should be observed that in such a configuration, the emitting source 1 is offset relative to the beam of width D that is finally radiated, and thus that the antenna is indeed an "offset" antenna.
This periscope type configuration having two reflectors 3 and 4 is used to reduce the dimensions of the active source 1, and, a priori, it is more advantageous than the simple configuration which could be provided by having an active source of size D equal to the size of the beam which would than be emitted directly.
It turns out, in practice, that the constraints applicable to the elements of the small-sized active source 1 are different from those that would apply to an equivalent large-sized active source used for directly radiating the beam of size D. Thus, in reality, in order to obtain the same performance, it is necessary to reduce the size of the elements of the source 1, and as a result to increase the number of adjustment or "control" devices associated with said source.
As a result the economic balance and the size characteristics of a conventional antenna as shown in FIG. 1 turn out, in contrast to what might have been expected a priori, not to produce any significant advantage over a simple active array antenna used for direct radiation.